© A. R. Dean

An adult Yellow-legged Gull displaying head-streaking.

It is not unusual for Mediterranean michahellis to have quite extensive head streaking immediately after the moult, from early August through September. Yellow-legged Gulls with well-streaked heads can suggest examples of the form atlantis, a designation which is sometimes assigned to birds from the west coast of Iberia as well as the Atlantic islands but is better restricted to birds originating from the Azores. True atlantis tends to have very concentrated, head-streaking, forming a dense hood, and also has - on average - a darker shade of grey on the mantle and brighter bare parts than Mediterranean michahellis. There can be subtle differences in the primary pattern e.g. a single mirror on p10 and a small black mark on p4.